1. Field of the Invention
The present invention relates to a coated titanium dioxide pigment with high ultraviolet (UV) absorption properties. This invention further relates to polymer compositions, especially polyvinyl chloride (PVC), incorporating coated titanium dioxide pigments with high UV absorption properties.
2. Description of the Related Art
Titanium dioxide (TiO2) is inherently photoactive, acting as a photocatalyst initiating chemical reactions which break down polymer resins. This type of degradation on the surface of a resin is evident in chalking and loss of surface gloss. Therefore, for many applications, titanium dioxide is surface-treated by coating/encapsulating the TiO2 particles to reduce photoactivity, that is, to make the particles “durable”. Surface treatments to encapsulate TiO2 particles include providing the particles with coatings of metal oxides, such as, silica, alumina, zirconia and the like, to suppress the photochemical activity. Such pigments are generally prepared by providing TiO2 pigment particles with a dense metal oxide coating. These pigments are used mainly in exterior applications or in uses that require some resistance to elements such as sunlight, temperature, humidity, etc. While dense metal oxide coatings enhance the durability of TiO2 pigment particles, there is still a need to limit the amount of UV light absorbed by the polymer.
U.S. Pat. Nos. 2,885,366 and 3,437,502 (also Re. 27,818) provide the basic teachings for making a durable titanium dioxide pigment having an amorphous silica coating followed by a coating of hydrous alumina. These patents are directed at providing durable TiO2 pigments for use in making paints.
There is further need for a TiO2 white pigment with improved UV absorption. UV absorption by TiO2 pigment protects polymer resins from direct UV degradation and discoloration. The higher the UV absorption of the TiO2 pigment, the greater the UV protection.
Titanium dioxide particles of small particle size, including “nano”-TiO2, are known to provide UV absorption when incorporated into polymer systems. However, due to their size, they can be beneficial for transparency but less beneficial for whiteness or opacity. In addition, small particles have large total surface area and require proportionately larger amounts of metal oxide surface treatment to suppress photocatalytic degradation inherent in TiO2.
Palmer, et al., in U.S. Pat. No. 5,352,725, disclose use of ultraviolet scattering and absorbing metal oxide particles, such as titanium dioxide and zinc oxide in polymer resins for the purpose of minimizing ultraviolet degradation of the resin and its underlying substrate. Palmer, et al., disclose use of particles in a size range of 0.001 micron to 0.20 micron, preferably, 0.01 micron to 0.15 micron, but are silent on how particle size is measured, and whether primary or agglomerates are being measured. Palmer teaches use of particles with low opacity to attenuate UV degradation in a surface layer to protect underlying polymer substrate from UV absorption.
Titanium dioxide of the prior art provides either good UV protection with transparency or low opacity, or it provides lower UV protection with good opacity. A titanium dioxide pigment combining both improved UV protection and good opacity is therefore desirable.
Polymer compositions tend to discolor, turning yellow, when exposed to UV light present in sunlight. The yellow coloring is unattractive and highly disfavored. Such compositions containing titanium dioxide pigments are widely used in building and residential construction materials, such as vinyl siding, door and window profiles, outdoor furniture and fencing. It is therefore, further desired to provide a polymer composition with improved UV stability.
The present invention meets these needs.